1. Field of the Invention
The present invention relates to a crane, in particular to an offshore crane, having a hoist rope winch, a hydraulic hoist winch drive and an overload protection which is connected to the hoist winch drive and which, when a predetermined permitted load limit is reached, permits a drawing off of the hoist rope from the hoist rope winch under a predetermined variable restoring force dependent on the radius and/or the permitted working load of the crane.
2. Description of the Prior Art
As a rule, cranes have an overload protection with a control device which prevents the crane from being moved into the overload range. In this process, the load acting on the crane is detected by means of a load detection device and is compared with a load limit. As soon as too great a load should be lifted or the radius should be increased so much by luffing down the crane boom with a lifted load that the load limit would be reached, the control device of the crane engages and sets the crane drives out of operation or only allows an actuation to the effect that the load acting on the crane is reduced. The load limit can be stored in the form of a load limit curve typical per se in dependence on the crane radius for various setup conditions. It has also already been proposed to define the load limit as a torque limit and always to make a current calculation of the load torque respectively acting on the crane from the respective radius and the winch rope force.
This type of overload protection is, however, not suitable for all possible load cases. Load cases can in particular occur with offshore cranes in which the mere switching off of the crane drives would result in a destruction of the crane. The loading and unloading of a ship, which is not carried out with calm water in a port, but at a higher sea state, is particularly dangerous here if the lifting hook of a crane standing on an offshore platform is attached onto too heavy a load which lies on a ship, an overload of the crane occurs when the ship sinks into a wave trough. The case is even more dangerous when the lifting hook of the crane catches on a component fixed to the ship body and the ship then sinks into a larger wave trough or moves away. The loads which occur would result in a destruction of the crane if the overload protection only sets the actuator units of the crane out of operation. To prevent a destruction of the crane in such cases, the hoist rope must be paid out.
It has already been proposed in such cases in which the load limit of the crane is reached by a pulling away of the lifting hook to actuate the hoist winch drive using the control device of the crane such that the hoist winch unwinds the hoist rope. The limited speed of the hoist winch drives used to date, however, restricts this form of overload protection to moderate wave conditions. With a very rough sea with high waves, overload cases can occur in which the ship to be loaded or unloaded sinks into a wave trough so fast that load peaks lying above the damage load of the crane are achieved.
It has furthermore already been proposed to provide a coupling between the hoist rope winch and the hoist rope winch drive motor and to decouple the hoist winch when the load limit is exceeded to permit the hoist rope to be paid out at high speed. In this connection, the retaining force of the hoist rope winch can be controlled using a hoist rope brake so that a hoist rope force is maintained with a decoupled coupling which corresponds to the load limit of the crane (cf. DE 202 19 282 U1). This solution with decoupling of the hoist rope winch and thus separation of the mechanical powertrain between the hoist winch drive and the hoist winch, however, occasionally encounters safety concerns. In addition, the coupling and brake to be installed and their controlling in dependence on various load and position sensors make the hoisting gear and its overload protection relatively complex.